rng terrain features in special rooms

[igroglaz]

/rfe make it possible to generate special rooms like this one:
https://media.discordapp.net/attachments/532644514574106624/923152448829415424/garden.png
using masks in dungeon.txt.. I suppose we could add to dungeon.txt new parameters like 'trees', 'water', etc - which could be used in special rooms